1. Field of the Invention
The present invention relates to a liquid crystal display device and an image shifting device. The liquid crystal display device and the image shifting device of the present invention can be used in an image display apparatus, e.g., a transmission type display apparatus such as a projector.
2. Description of the Background Art
Liquid Crystal Display Device
Japanese Laid-Open Patent Publication Nos. 2-55322 and 9-96824 disclose techniques for improving the response speed of a liquid crystal panel by increasing the temperature of the liquid crystal material with a transparent heater provided in the liquid crystal panel. With these techniques, some improvement can be expected in the response speed of the liquid crystal material by heating the liquid crystal panel. However, an increase in the panel temperature may lead to deterioration of the liquid crystal property of the liquid crystal composition, and the anisotropy of the liquid crystal material may be lost near the nematic-isotropic phase transition temperature (TNI), whereby a sufficient display quality cannot be maintained.
Japanese Laid-Open Patent Publication No. 8-171084 discloses a technique for controlling the temperature of a liquid crystal panel while applying a voltage across the liquid crystal material so as to maximize the contrast ratio of the displayed image. With this technique, deterioration of the displayed image can be prevented to some degree. However, this publication fails to provide clear guidelines as to how the panel temperature should be controlled for various panels using various liquid crystal materials and specifications. Moreover, the configuration of the heating panel and the control system therefor is very complicated. Therefore, the disclosed technique cannot easily be applied to many types of liquid crystal display apparatuses.
Japanese Laid-Open Patent Publication No. 2001-83480 discloses a technique for controlling the temperature of a liquid crystal panel. Specifically, the publication discloses a cholesteric liquid crystal display apparatus in which the cholesteric liquid crystal material is driven at a temperature that is higher than room temperature and lower than the isotropic phase temperature aiming at increasing the speed of the orientation transition of the cholesteric liquid crystal material. However, it is difficult to apply such a temperature controlling technique to liquid crystal display apparatuses of a type in which the selective reflection of a cholesteric liquid crystal material is controlled with an electric field.
In recent years, the response speed, the brightness and the contrast ratio of liquid crystal display devices have been increased rapidly, and there has been a demand for liquid crystal display apparatuses and projection type liquid crystal display apparatuses capable of displaying moving images while ensuring a high display quality.
Image Shifting Device
U.S. Pat. No. 6,061,103 and Japanese Laid-Open Patent Publication No. 8-194207 disclose a technique for realizing a high definition image display by optically shifting red, green and blue pixels from one another using an image shifting device to form a combined image in a time-division manner. The combination of a liquid crystal device and a birefringence device is proposed as the image shifting device. In this way, the apparent resolution can be tripled. Although each pixel only displays one of red, green and blue without the image shifting (wobbling) using the image shifting device, it is possible to realize a full color display with an improved image resolution by shifting images of different colors to form a combined image in a time-division manner.
As for the method for driving an image display apparatus, an interlaced driving method, a non-interlaced driving method, and the like, are widely employed. In an interlaced driving method, odd-numbered rows and even-numbered rows are selected alternately field by field to form a combined image from an odd-numbered field and an even-numbered field. The selection time for each field is typically 16.6 ms (milliseconds) (60 Hz). In the non-interlaced driving method, rows of an image is selected successively irrespective of whether they are odd- or even-numbered rows. The selection time for each field is typically 16.6 ms (60 Hz) also in the non-interlaced driving method. The term “field” refers to a vertical sync period, and with liquid crystal display apparatuses, the scanning period including the blanking period corresponds to the field period.
In the image display apparatus disclosed in U.S. Pat. No. 6,061,103 and Japanese Laid-Open Patent Publication No. 8-194207, each field period is divided into “subfield periods” corresponding to the red, green and blue pixel shift positions, and a different image is displayed in each subfield period. The image shifting operation by the image shifting device needs to be synchronized with the timing at which a subfield is switched to another. Since the subfield period obtained by dividing one field period is as short as about 5 ms, it is important that the polarization of the liquid crystal device transitions quickly upon switching of subfields.
However, when a conventional liquid crystal device is used, the response time may be longer than the subfield selection time, or the rising response time (τon) may be different from the falling response time (τoff), whereby the timing at which the image shifting operation is performed may be shifted. When the image shift timing is shifted, a double image phenomenon occurs, and flicker occurs due to the double image phenomenon. In such a case, the image quality is deteriorated substantially.
As described above, for image display apparatuses that use the wobbling technique, it is necessary to improve the response speed of the liquid crystal device of the image shifting device, and it is also necessary for the image-displaying liquid crystal display device to have a high response speed that matches the subfield period to be selected. Therefore, when a conventional liquid crystal device is used, further improvements in the image quality are required.
By using two or more sets of liquid crystal devices and birefringence devices, it is possible to obtain an image shifting device having three or more shift positions. In such an image shifting device, it is necessary that the liquid crystal devices have a uniform response speed, which is not required for an image shifting device using only one liquid crystal device. If each liquid crystal device has a different response time, the image shift timing will be varied, thereby deteriorating the image quality significantly. Therefore, it is necessary to maintain a uniform temperature for all the liquid crystal devices.
Moreover, it is very important to design a liquid crystal device in view of the panel temperature and the temperature characteristics of the liquid crystal composition so that the liquid crystal device provides a high quality and a high response speed for various liquid crystal panel structures and display modes. Normally, the display properties related to the image quality such as the brightness and the contrast deteriorate by simply heating the liquid crystal panel. In addition, if one attempts to realize a high response speed that matches the subfield period only for the liquid crystal devices of the image shifting device, it is difficult to realize an image display apparatus having a high quality and a high definition. Thus, it is important that the liquid crystal display device for modulating light from the light source according to input signals be capable of displaying image information in synchronization with the subfield period so as to guide the image information to the image shifting device. Therefore, it is required to uniformly control the temperature of the liquid crystal display device and that of the image shifting device.
Japanese Laid-Open Patent Publication No. 11-326877 discloses an image display apparatus that incorporates a mechanism for adjusting the temperature of the image shifting device (vibration means) for the purpose of improving the response speed of the liquid crystal device. Specifically, the publication discloses providing a sheet heater around the liquid crystal device of the image shifting device to directly heat the liquid crystal device. The publication also discloses other approaches, e.g., making the transparent conductive film of the liquid crystal device so that the transparent conductive film itself functions as a heater, and providing a transparent heater pattern on the liquid crystal device. However, it fails to disclose any of the possible problems confronted when adjusting the temperature of the image shifting device.